Preparation of alkylfluosilanes by the reaction of fluosilicates with aluminum trialkyls



ready at lower temperatures, such as at 100 C.

Uitedl States PREPARATIQN F ALKYLFLUOSILANES BY THE REACTION OF FLUOSILICATES WITH ALUMI- NUM TRIALKYLS No Drawing. Application December 12, 1957 SerialNo. 702,274

. Claims priority, application Germany June 11, 1953 4 Claims. (Cl. 260-4482) The invention relates to the preparation. of alkylfluosilanes and alkyl. silanes, and this application is a continuation-in-part of my copending application, Serial No. 434,063, for a process formanufacturing organo-silicon compounds, filed June 2, 1954, now abandoned.

In said application, I have disclosed the reaction of silicon tetrafluoride and organo-ruosilanes at elevated temperatures with aluminum alkyls or etherates thereof, whereby the fluorine atoms of the silicon-fluorine compounds are exchanged against alkyl groups of the aluminum alkyl withformatic-n of alkyl fluosilanes and alkyl silanes. As or anic groups of the organo-fluosilanes, any monovalent hydrocarbon radical may be employed; for reasons of commercial availability, the lower alkylfiuosilanes will .be generally used as starting materials. For similar reasons, the lower aluminum alkyls, such as trimethyl and triethyl aluminum, are preferably employed. Instead of the aluminum trialkyls, also the dialkyl aluminum monofluorides may be employed.

The best temperature range is about 140 to 260 C. Under favorable conditions, the reaction may. start al- If a solvent like ether is used, the temperature may be gradually raised to 260 C., whereby said solvent is distilled off. Temperatures up to about 300 C. may. also beused, though at high temperatures the alkylation degree of the silane may be reduced, particularly when the alkylation is carried out with higher aluminum alkyls.

The reaction is carried out under atmospheric pressure, but superatmospheric or reduced pressures may also be applied.

A preferred embodiment of the invention consists in using silicon fluoride in the form of a salt of fluosilicic: acid. Suitable salts are, for instance, the anhydrous alkali metal salts such as rosin, and Na SiF and it is of particulad advantage to employ freshly prepared salts. Also, the salts of the alkaline earth metals, such as BaSiF may be used.

As aluminum alkyls, I use trimethyl or triethyl aluminum; higher alkyls are less useful because they increase in the reaction product the ratio of silicon-hydrogen compounds, wherein fluorine has been replaced by hydrogen.

The formation of silicon-hydrogen compoundsis negligible when trimethyl aluminum and low temperatures are used; small amounts, in the order of magnitude of about 23%, are formed with triethyl aluminum in the optimum temperature range of 2l0-230 C., and these amounts increase with increasing temperature.

in the process, a mixture of alkyl silane and fluorinated alkyl silanes is obtained, which is readily fractionated because of the great diflerence in the boiling points, which is, in the fluorine series, much greater than in the corresponding chlorine series.

The boiling points of the fluorides of the ethyl silane group, for instance, are as follows:

atent SiCl C H 98" c. SiC1 (C H 129 c. SiCl(C H 143 csuc n 151 c- In comparison therewith, the boiling points of the corresponding chlorine series are:

it will be noted that the boiling point difference between SiF (C H and SiF(C H is 114 C., where the same difference between the corresponding chlorides is only 55 C. In the case of the methyl compounds,'the diiferences are 46.4 C. and 8.4 C., respectively.

A'further advantage of the fluorides over the chlorides is that in the methylchlorsilane synthesis, the generation of SiCL; can hardly be avoided, which has almost the same boiling point (56 C.) as (CH SiCl(57.3 C.).

SiF if it is formed at all in the alkylation of the fluosilicates, does not interfere with the recovery of the fluosilanes, because it sublimes already at a temperature of .96 C.

On heating, for instance, sodium fluosilicate with triethyl aluminurmthe following reactions take place:

+3 Si( (3 1-1 4AlF 6NaF.

All these reactions take place concurrently, but it is possible to favor one reaction overthe others by selecting the ratio of the reactantsin accordance with the corresponding equation. As dihalodia kyl silanes are preferred starting materials for the manufacture of silicones, the reactants of the invention will be preferably reacted in amounts corresponding to Equation 2 so as to obtain an optimum yield of the difluoro compound.

I have found that the formation of the difluoro compounds is further increased when-the reaction is carried out in an organic diluent which acts assuspending medium for the fluosilicic salt and as diluent or solvent for the aluminum alkyl. Any organic liquid is, suitable which does not react with the reactants and reaction products and has a boiling point at least about 40? C. above the boiling point of the aluminum alkyl, such as hydrocarbons not containing olefinic-doublebonds and boiling between'about 180 C. and 450 C.- .Such hydrocarbons are, for instance, paraflins having at least-9C atoms, methyl naphthaline, and particularly high boiling mineral oils. Also tetra-alkyl silanes of a suitable boiling range may be employed.

In such suspension, which can be easily stirred from the start to the completion of the reaction, the aluminum alkyl readily reacts completely with the fluosilicate.

The diluent is preferably employed in an-amount by weight approximately corresponding to the amount of the reacting fluosilicate; however, also smaller amounts (about 50 to may already exert a favorableeffect.

A preferred embodiment of theinvention consists in heating,.with,vigorous stirring, a suspension ofsodium fluosilicate in a mineral oil at 200-25t0 C. and thenradding slowly thealuminum alkyl. .Theforrned mixture of alkyl fluosilanes and tetra-alkyl silane distills ,otf continuously, conforming to the rate of addition; of the aluminum alkyl. It is of advantage to introducethe aluminum alkyl below the surface of the suspension.

Fatented Jan. 19,, 1,960

A It is also possible to add first the aluminum alkyl to the heated diluent and to introduce then the fluosilicate. After completion of the reaction, the diluent can be eparated from the NaFAlF residue by distillation, us decantation or filtration.

If an expensive diluent has been used'which is to be recovered as completely as possible, the AlF -Naf-diluent residue may be mixed with a low boiling solvent, such as hexane or the like; after separation of the solid phase from the diluent-solvent mixture, the low boiling solvent 'is readily distilled off from the diluent, such as mineral oil, tetrapropyl silane, and the like, which diluent is then ready to be'used again in a subsequent operation.

The NaFAlF residue may be processed to form Na SiF by admixing it with SiG and heating it at 1,000-

' 1,200 C. In this way, SiF is generated, which is reacted with NaCl to form Na SiF Another way to produce SiF from the NaFAlF residue consists in the reaction with SiO and concentrated H SO or S at 350 to 450, C.

In the hydrolysis of the alkylfiuosilanes to silicones, HF or alkali metal or ammonium fluoride is obtained,

which too may be converted with SiO and H 50; into amples, where no diluent was used. All parts are given by weight.

Example 1 A mixture of 228 parts (2 moles) of triethyl aluminum and 564 parts (3 moles) of Na SiF was heated on an oil bath at 200250? C. 256 parts of a mixture were obtained which consisted of 18% SiF C H 42% SiF (C H 15% .SiF (c n Si 0 m Example 2 188 parts (1 mole) of Na siFg were suspended in 300 parts ofa mineral oil having a low content of sulfur and trimethylaluminurn, was quantitative.

olefins and boiling at about 180-240 C. under a pressure of 0.1 mm. (as available in commerce, for instance under the trade names Bayol 85 or Viscobiloil SERA 4), and heated at about 230 C., under stirring. Then 76.6

parts of Al(C H were slowly introduced dropwise into the suspension.

' The reaction started at once, whereby the obtained 26 parts=23.l% SiF (C H 75 PartS=60-7% 10.6 parts= 8.6% SiF (C I-I 9.4 7.6% Si (C H The yield, calculated on triethyl aluminum, was 97%.

The reaction product contained about 34% of partially ethylated silanes (SiH (C H etc.). The ratio of such partially ethylated silanes over the corersponding .fluorinated silanes increases with increasing reaction temperature. At a temperature of 280 C., for instance,

their amount increases to about 15 to 20% of the total 48 parts reaction.

Example 3 In the same manner as set forth in Example 1, 423 parts of Na SiF were heated with 508 parts of triethyl aluminum ethyl etherate. There were obtained:

61 SiF (C H5)2 73.6 parts=27.9% SiF (C H 80.4 parts=30.5% Si (C H The total yield, calculated on the sodium fluosilicate, was 79 percent; 6 percent of the triethyl aluminum etherate had not reacted.

Example 4 I 153.5 parts of Na SiF were suspended in parts of mineral oil (b =2l0 C.) and heated-at 230235 C. Over a period of 15 hours, 102.4 parts of Al(C H O(C H were added to the suspension. By fractionated distillation there were obtained 11 parts of SiF (C H (=59.2%) of SiF (C H 14 parts of SiF(C H and ,8 parts of Si(C H Example 5 598 parts of triethyl aluminum-ethyl etherate were mixed with 1150 parts of sodium fiuosilicate and slowly heated on anoil bath to 220 C., whereby the ethyl ether distilled off.

The reaction took place at about 210220 C. and was terminated after 5 minutes.

The following compounds were obtained:

SiF3(C2H5) parts=12.3%

SiF (C H 63 parts=15.1% Si (C2H5)4 PZItS=19.7%

The total yield, calculated on Na siFg, was 86%.

Example 6 144 parts of trimethyl aluminum were passed into a suspension of 570 parts sodium fluosilicate in 500 parts of a high boiling mineral oil at a reaction temperature of 200-240" C. with stirring. The reaction was carried out in a closed reactor, whereby the formed mixture of methylfluosilanes wasreleased at atmospheric pressure from a reflux cooler. Theconversion, calculated on The content of dimethyl difluosilane in the mixture of methyl fiuosilanes was 62 percent.

It the reaction is not carried out in a closed vessel, it is or" advantage to use trimethyl aluminum in an excess of about 20-40 percent, because port of the. trimethyl aluminum distills oil with the mixture of the methyl fluosilanes. After separation from said mixture, the recovered trimethyl aluminum may be again used for the Example 7 A batch of triethyl aluminum ethyl ether was heated to about 140-230 C., and a current of dry oxygen-free SiF 'was passed into said batch, whereby the gas was introduced below the surface of the liquid. Immediate reaction took place, and ether and a mixture of ethyl-- fluosilanes and ethyl silane distilled oif. Said mixture contained more than 50 percent of SiF C H If the reaction is carried out in an autoclave with an excess of about 50 percent of triethyl aluminum, almost pure tetraethyl silane is obtained.

In both cases, the conversion of SiF is almost quantitative.

I claim:

1. A process for preparing alkylfluosilanes comprising heating a salt of fluosilicic acid selected from the group consisting of alkali metal fiuosilicates and alkaline earth metal fluosilicates at a temperature of about 100 to 300 C. under anhydrous conditions with a lower aluminum trialkyl in an inert liquid organic diluent having a boiling point of at least 40 C. above the boiling point of said aluminum trialkyl and being selected from the group consisting of hydrocarbons not containing olefinic double bonds and tetraalkyl silanes.

2. The process as defined in claim 1, wherein sodium fiuosilicate is used.

3. A process for preparing alkyl fluosilanes comprising heating sodium fluosilicate in a mineral oil, which is substantially free from sulfur and oxygen-containing compounds and has a boiling point of at least 180 C. at a temperature of about 140 to 300 C. and at least about 40 C. below the boiling temperature of said mineral oil,

with an organo-aluminurn compound selected from the 15 group consisting of AIR}; and A1R -(C H O 6 wherein R is a member of the group consisting of methyl and ethyl.

4. The process as defined in claim 3, wherein about 3 moles of sodium fluosilicate are reacted with 2 moles of 5 the organo-aluminum compound.

References Cited in the file of this patent UNITED STATES PATENTS 2,436,777 Pletcher et a1. Feb. 24, 1948 2,762,824 Brown Sept. 11, 1956 FOREIGN PATENTS 1,038,184 France May 6, 1953 K26,023 Germany Sept. 27, 1956 OTHER REFERENCES Soshestvenskaya: Jour. Gen. Chem. (USSR), vol. 10 (1940), pp. 1689-93. 

1. A PROCESS FOR PREPARING ALKYLFLUOSILANES COMPRISING HEATING A SALT OF FLUOSILICIC ACID SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL FLUOSILICATES AND ALKALINE EARTH METAL FLUOSILICATES AT A TEMPERATURE OF ABOUT 100 TO 300* C. UNDER ANTHYDROUS CONDITIONS WITH A LOWE ALUMINUM TRIALKYL IN AN INERT LIQUID ORGANIC DILUENT HAVING A BOILING POINT OF AT LEAST 40*C. ABOVE THE BOILING POINT OF SAID ALUMINUM TRIALKYL AND BEING SELECTED FROM THE GROUP CONSISTING OF HYDROCARBONS NOT CONTAINING OLEFINIC DOUBLE BONDS AND TETRAALKYL SILANES. 